The treatment of a cancer is one of applications of radiation beam. Recently, particle beam therapy in which a heavy particle beam such as a proton beam or a carbon beam is irradiated on the cancer cell has been attracted attention. Referring to FIGS. 14 to 16, the characteristic of a particle beam irradiation in which the particle beam is irradiated to kill a cancer cell will be described. In a case where thin diameter pencil beams of various kinds of radiation beams are irradiated on a human body, the dose distribution of the radiation beam in the human body changes as shown in FIG. 14. As shown in FIG. 14, among various kinds of radiations, a radiation beam with a small mass, such as an X-ray or a gamma ray, has a relative dose which becomes maximum in a portion close to the surface of the body, and is decreased as the depth from the surface of the body is increased. On the other hand, a particle beam with a large mass, such as a proton beam or a carbon beam, has a relative dose which has a peak value at a position where the beam stops at a deep portion from the surface of the body, that is, immediately before the range of the particle beam. This peak value is called the Bragg Peak (BP).
Particle beam cancer treatment is such that this Bragg peak BP is irradiated to a tumor formed in a human organ and the treatment of the cancer is performed. In addition to the cancer, it can also be used for a case where a deep portion of a body is treated. A region to be treated, including a tumor, is generally called an irradiation target (TV). The position of the Bragg peak BP is determined by the energy of an irradiated particle beam, and as the energy of the particle beam becomes high, the Bragg peak BP is formed at a deep position. In the particle beam treatment, it is necessary that the particle beam is made to have a uniform dose distribution over the whole of the irradiation target to be irradiated. In order to give the Bragg peak BP to the whole region of the irradiation target, “spread of irradiation field” of the particle beam is performed.
This “spread of irradiation field” is performed in three directions of an X-axis, a Y-axis and a Z axis perpendicular to each other. When the irradiation direction of the radiation beam is made the direction of the Z-axis, the “spread of irradiation field” is first performed in the Z-axis direction. The “spread of irradiation field” in the irradiation direction of the radiation beam is generally called depth direction irradiation field spread. The second “spread of irradiation field” is such that the irradiation field spread is performed in the X-axis and Y-axis directions, and since the irradiation field spread is performed in the lateral direction perpendicular to the depth direction, it is generally called lateral direction irradiation field spread.
The depth direction irradiation field spread is performed to spread the Bragg peak BP, which is in the irradiation direction of the particle beam, in the depth direction since the width of the Bragg peak BP in the irradiation direction of the particle beam is narrow as compared with the extent of the irradiation target in the depth direction. On the other hand, the lateral direction irradiation field spread is performed to spread the Bragg peak BP in the direction perpendicular to the irradiation direction since the diameter of the particle beam is smaller than the size of the irradiation target in the direction perpendicular to the irradiation direction. With respect to the depth direction irradiation field spread and the lateral direction irradiation field spread, various kinds of methods have been proposed so far. Recently, a spot scanning technique has attracted attention.
In spot scanning technique, as a lateral direction irradiation field spread method, there is a method in which a deflection electromagnet provided at the upstream portion of a particle beam irradiation part of a particle beam irradiation apparatus is used to scan the particle beam in the XY plane, and the irradiation position of the particle beam is moved with the lapse of time to obtain a wide irradiation field. In this method, a uniform dose distribution can be obtained by suitably overlapping adjacent irradiation spots of small diameter pencil beams. Scanning methods of a pencil beam include a raster method of performing scanning continuously with respect to time, and a spot method of performing a step-like scanning with respect to time.
As the depth irradiation field spread method, there is a method in which the energy of the particle beam itself irradiated from a particle beam irradiation apparatus is controlled. In this method, the energy of the particle beam is controlled by changing the acceleration energy of an accelerator to accelerate the particle beam, or the energy of the particle beam is changed by inserting a tool called a range shifter so as to cross the particle beam. There is also a method in which both the control by the accelerator and the range filter are used.
In the depth direction irradiation field spread method, the particle beam is made the beam having the energy of specified intensity, and after the Bragg peak BP with a uniform dose is irradiated to one irradiation layer of the irradiation target, the energy of the particle beam is changed, and the Bragg peak BP is irradiated to next irradiation layer in the irradiation target TV. Such operation is repeated plural times, and the Bragg peak BP of the particle beam is irradiated to the plural irradiation layers, so that the spread-out Bragg peak SOBP having a desired width in the beam irradiation direction can be obtained. The depth direction active irradiation field spread method is a method in which the energy of the particle beam is changed in the state where the particle beam is not moved in the X and Y-axis directions and is fixed to a definite irradiation position.
In order to obtain the spread-out Bragg peak SOBP having the desired width, it is necessary to suitably adjust the dose of each irradiation layer of the irradiation target TV. FIG. 15 shows an example of dose distribution of each irradiation layer. In FIG. 15, the vertical axis indicates the relative dose, and the horizontal axis indicates the depth in the body. Each of plural curve lines indicated by broken line indicates the depth direction dose distribution of each beam having different energy, that is, the dose distribution of each irradiation layer, in a case where beams having different energy are irradiated. The dose that is obtained by integrating the dose of each irradiation layer is the dose which is applied to the depth direction, and is shown by a solid curve line in FIG. 15. The solid curve line is the spread-out Bragg peak obtained by the above-mentioned irradiation field spread method. A particle beam irradiation method in which the lateral direction irradiation field spread method and the depth direction irradiation field spread method are combined is known as the spot scanning technique.
On the other hand, in a treatment of cancer, it is necessary to kill a cancer cell without affecting normal cells. The details will be described referring to FIG. 16. In FIG. 16, the horizontal axis indicates the dose of particle beam, a dotted line indicates the death ratio of tumor cell to the dose and an alternate long and short dash line indicates the ratio of adverse effect of normal cell to the dose. In a treatment of cancer, it is necessary to increase the death ratio of tumor cell and to decrease the ratio of adverse effect to normal cell. The cure ratio is maximum at the dose where the difference between the death ratio of tumor cell and the ratio of adverse effect to normal cell is large (the dose that is shown as an optimal dose in FIG. 16). Further, since normal cells exist outside of therapeutic area, it is necessary to decrease the dose to be irradiated outside of therapeutic area as much as possible. That is, it is necessary to make the dose distribution in a boundary region to be steep. In a spot scanning technique, in order to make the dose distribution in a boundary region steep, the technology to make the spot size (beam diameter) of a particle beam to be irradiated on a boundary region small was proposed (for example Patent Document 1).
In order to compensate the deviation of irradiation caused by moving the position of diseased site due to the respiration of the patient, a method in which irradiation is performed plural times at the same spot in each irradiation layer for time-dividing was proposed (for example, Patent Document 2, FIG. 11). Further, in Patent Document 2, a technique for controlling the irradiation dose synchronized with the respiration phase, in view of the moving of the position of diseased site due to the respiration of the patient, was proposed.